Cooling system for fuel cells

ABSTRACT

A fuel cell stack assembly comprises a stack of fuel cells, each fuel cell having a cooling air conduit with an input/output ventilation aperture disposed on a ventilation face of the stack. The ventilation apertures form an array over said ventilation face of the stack. A first fan is configured to direct air flow through a first portion of the ventilation face and a second fan is configured to direct air flow through a second portion of the ventilation face. A reconfigurable plenum is in fluid communication with the first fan and the second fan and has a first configuration in which air is directed, by the first and second fans, through the first and second portions of the ventilation face in the same direction, and a second configuration in which air is directed, by at least one of the fans, respectively through the first and second portions of the ventilation face in opposing directions. When operating in the second configuration, the directions of air flow through the first and second portions of the ventilation face are periodically reversed.

RELATED

This patent application claims priority to International PatentApplication PCT/GB2013/051596, filed Jun. 19, 2013, and United KingdomPatent Application GB1210880.9, filed Jun. 20, 2012, the contents ofwhich are incorporated by this reference as if fully set forth herein intheir entirety.

FIELD

The present invention relates to electrochemical fuel cells disposed ina stack formation, and in particular to cooling systems for such fuelcell stacks.

BACKGROUND

Conventional electrochemical fuel cells convert fuel and oxidant,generally both in the form of gaseous streams, into electrical energyand a reaction product. A common type of electrochemical fuel cell forreacting hydrogen and oxygen comprises a polymeric ion transfermembrane, also known as a proton exchange membrane (PEM), within amembrane-electrode assembly (MEA), with fuel and air being passed overrespective sides of the membrane. Protons (i.e. hydrogen ions) areconducted through the membrane, balanced by electrons conducted througha circuit connecting the anode and cathode of the fuel cell. To increasethe available voltage, a stack is formed comprising a number ofseries-connected MEAs arranged with separate anode and cathode fluidflow paths. Such a stack is typically in the form of a block comprisingnumerous individual fuel cell plates held together by end plates ateither end of the stack.

Because the reaction of fuel and oxidant generates heat as well aselectrical power, a fuel cell stack requires cooling once an operatingtemperature has been reached, to avoid damage to the fuel cells. Coolingmay be achieved by forcing air through the fuel cell stack. In an opencathode stack, the oxidant flow path and the coolant flow path are thesame, i.e. forcing air through the cathode fluid flow paths bothsupplies oxidant to the cathodes and cools the stack.

However, optimal operation of the fuel cell stack relies on maintainingthe fuel cells at their optimal operating temperature and fuel cellstack efficiency can be adversely affected at low ambient temperaturesor when a stack is starting up from cold. Thus, it is desirable to beable to regulate the cooling efficiency of air flows through thecathode.

One technique for achieving this is to recycle some or all of theexhaust air from a fuel cell stack that has been passed over thecathodes back to the stack air input. The exhaust air is preheated byits first passage through the stack, and a duct takes this exhaust airaround to the front of the stack to re-use, possibly mixed with aproportion of cool air, thus reducing the overall cooling efficiency andallowing the fuel cell stack to run efficiently at low ambienttemperatures. A potential disadvantage of this arrangement is thatextensive ducting is required to pass air from an output face of thefuel cell stack, right around the stack to the input face. This adds tothe bulk of the fuel cell system and limits the amount of space forother support systems to be built on to the fuel cell stack.

A further potential disadvantage of this recirculating arrangement isthat the recirculated warm air, when mixed with very cold ambient air,can cause substantial condensation to occur at the inlet to the fuelcell stack.

DISCLOSURE

It is an object of the present invention to provide an alternativearrangement for providing a degree of control over the coolingefficiency of air flows through a fuel cell stack.

According to one aspect, the present invention provides a fuel cellstack assembly comprising:

-   -   a stack of fuel cells, each fuel cell having a cooling air        conduit with an input/output ventilation aperture disposed on a        ventilation face of the stack, the ventilation apertures forming        an array over said ventilation face of the stack;    -   a first fan configured to direct air flow through a first        portion of the ventilation face and a second fan configured to        direct air flow through a second portion of the ventilation        face;    -   a reconfigurable plenum in fluid communication with said first        fan and said second fan, the plenum having a first configuration        in which air is directed, by the first and second fans, through        the first and second portions of the ventilation face in the        same direction, and a second configuration in which air is        directed, by at least one of the fans, respectively through the        first and second portions of the ventilation face in opposing        directions.

The plenum may be automatically reconfigurable as a function ofoperating temperature of at least a portion of the fuel cell stackand/or time of operation. The reconfigurable plenum may comprise: afirst end proximal to said first and second fans and a second end distalto said first and second fans switchable between an open configurationand a closed configuration, the open configuration facilitating exit ofair flow from the plenum at the second end and the closed configurationforcing return of at least some air from the first fan to the secondfan. The reconfigurable plenum may be reconfigurable in a plurality ofintermediate configurations between the open configuration and theclosed configuration each intermediate configuration forcing return ofdifferent proportions of air from the first fan to the second fan. Thereconfigurable plenum may include a variable occluding member at thesecond end of the plenum. The fuel cell stack assembly may include a fancontroller configured to drive the first and second fans in opposingdirections when the reconfigurable plenum is in the secondconfiguration. The fuel cell stack assembly may include a fan controllerconfigured to drive the first fan and shut down the second fan when thereconfigurable plenum is in the second configuration. The fuel cellstack assembly may include a plurality of said first fans and aplurality of said second fans, each of the first and second fanscooperating with a said reconfigurable plenum. The first fans and thesecond fans may be arranged in groups, each group cooperating with onesaid reconfigurable plenum. The first portion of the ventilation faceand the second portion of the ventilation face may correspond todifferent parts of the same cells. The first fan and the second fan maybe adjacent one another and adjacent the ventilation face. The fuel cellstack assembly may include a first reverse operating fan configured todirect air flow through the first portion of the ventilation face in adirection opposite to that of the first fan and a second reverseoperating fan configured to direct air flow through the second portionof the ventilation face in a direction opposite to that of the secondfan. The fuel cell stack may include a control system adapted to, whenthe system is operating in the second configuration, periodicallyreverse the directions of air flow through the first and second portionsof the ventilation face.

According to another aspect, the present invention provides a method ofoperating an air cooled fuel cell stack in which each fuel cell in thestack has a cooling air conduit with an input/output ventilationaperture disposed on a ventilation face of the stack, the ventilationapertures forming an array over said ventilation face of the stack, theventilation face having a first portion and a second portion, the methodcomprising:

-   -   in a first mode of operation, ventilating the stack using a        first fan and a second fan, the first fan directing air through        a first portion of the ventilation face and the second fan        directing air through a second portion of the ventilation face,        the air flow through the first and second portions being in the        same direction;    -   in a second mode of operation, ventilating the stack using at        least the first fan to direct air through the first portion of        the ventilation face in a first direction and through the second        portion of the ventilation face in a second direction opposite        to the first direction.

In the second mode of operation, the stack may be ventilated using thefirst fan to direct air flow through the first portion of theventilation face in said first direction and using the second fan todirect the air through the second portion of the ventilation face insaid second direction opposite to the first direction. Operation of thefuel cell stack ventilation may be switched between the first and secondmodes of operation by reconfiguring a reconfigurable plenum in fluidcommunication with said first fan and said second fan, thereconfigurable plenum having a first end proximal to said first andsecond fans and a second end distal to said first and second fans whichis switchable between an open configuration and a closed configuration,the open configuration facilitating exit of air flow from the plenum atthe second end and the closed configuration forcing return of at leastsome air from the first fan to the second portion of the ventilationface. Automatic switching between the first and second modes, as afunction of operating temperature of the fuel cell and/or time ofoperation, may be provided. When operating in the second mode ofoperation, the directions of air flow through the first and secondportions of the ventilation face may be periodically reversed.

DRAWINGS

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows an exploded perspective view of louver panel, fancontainment box and air filter box components of a ventilation systemfor a fuel cell stack;

FIG. 2 shows a perspective view of the assembled louver panel and fancontainment box of FIG. 1, from the reverse side, in ahorizontally-banked, lower bank vented configuration;

FIG. 3 shows a perspective view of the assembled louver panel and fancontainment box of FIG. 1, from the reverse side, in ahorizontally-banked, upper bank vented configuration;

FIG. 4 shows a perspective view of the assembled louver panel and fancontainment box of FIG. 1, from the reverse side, in a fully closedconfiguration;

FIG. 5 shows a perspective view of the assembled louver panel and fancontainment box of FIG. 1, from the reverse side, in ahorizontally-banked, full recirculation configuration;

FIG. 6 shows a perspective view of the assembled louver panel and fancontainment box of FIG. 1, from the reverse side, in ahorizontally-banked, fully open configuration;

FIG. 7 shows a cross-sectional perspective view of the assembled louverpanel and fan containment box of FIG. 1, from the reverse side, in thehorizontally-banked, full recirculation configuration of FIG. 5;

FIG. 8 shows a schematic cross-sectional view of the louver panel andfan containment box showing three positions of the louvers;

FIGS. 9 a and 9 b show schematic cross-sectional views of a fuel cellstack with a ventilation system including an alternative configurationof louver panel in (a) a fully open, non-recirculating configuration and(b) a fully recirculating configuration; and

FIG. 10 shows a schematic cross-sectional view of a fuel cell stack witha ventilation system including an alternative configuration of louverpanel in a partially open, partially recirculating configuration.

FURTHER DISCLOSURE

The present invention will now be described in relation to air-cooled“open cathode” fuel cell technology in which the cooling air flow passesdirectly through cathode flow channels that provide oxidant to the MEA.However, the principles described here can be deployed more generallyfor fuel cell air cooling, e.g. where the cooling air flow is notnecessarily the same as the oxidant flow.

With reference to FIG. 1, a ventilation assembly 1 for a fuel cell stack(not shown) comprises louver panel 2, a fan containment box 3 and an airfilter box 4. The louver panel 2 has a number of louvers 5 which can bevaried in their angular disposition or position so as to vary a flow ofair through the panel 2. The fan containment box 3 has an array 6 offans 7 each housed within an air guide chamber 8. The filter box 4comprises one or more a filter units 9. The louver panel 2, fancontainment box 3 and air filter box 4 can be assembled in a number ofways, together with or around further components such as a fuel cellstack, as will become apparent later.

FIG. 2 shows parts of a ventilation assembly 20 similar to that shown inFIG. 1 viewed from the other side. The ventilation assembly 20 is shownin a horizontally-banked, lower bank vented configuration, in which thefans 7 are grouped horizontally in banks 21 a-21 d. Each bank 21 has alouver 5 and the louvers have been labelled respectively 5 a-5 d. It canbe seen that louvers 5 a and 5 c are in fully closed positions, i.e.vertical as drawn, whereas louvers 5 b and 5 d are in a fully openposition, i.e. horizontal as drawn. Each 21 bank is separated by apartition 22 respectively labelled 22 a-22 c. As will be described inmore detail later, partitions 22 a and 22 c have a wedge shape on eachface whereas central partition 22 is flat on each face. A set of controlelements 23 is provided on a side face 24 of the ventilation assembly tocontrol the disposition of the louvers 5 a-5 d.

FIG. 3 shows the ventilation assembly 20 in a horizontally-banked, upperbank vented configuration, in which the fans 7 are grouped horizontallyin banks 21 a-21 d. It can be seen that louvers 5 b and 5 d are in fullyclosed positions, i.e. vertical as drawn, whereas louvers 5 a and 5 care in a fully open position, i.e. horizontal as drawn. The set ofcontrol elements 23 on side face 24 is repositioned compared with FIG. 2consistent with the different disposition of the louvers 5 a-5 d.

FIG. 4 shows the ventilation assembly 20 in a fully closedconfiguration, in which the louvers 5 a to 5 d are all in fully closedpositions, i.e. vertical as drawn. The set of control elements 23 onside face 24 is repositioned compared with FIGS. 2 and 3 consistent withthe different disposition of the louvers 5 a-5 d.

FIG. 5 shows the ventilation assembly 20 in a full recirculationconfiguration, in which the louvers 5 a-5 d are all in a recirculationposition which is oblique to both the vertical (closed) position and thehorizontal (open) positions of FIGS. 2 to 4. The precise angle at whichthe louvers are positioned to achieve the full recirculationconfiguration is dependent on the wedge shape of partitions 22 a and 22c as will be better described later in connection with FIGS. 7 and 8.The set of control elements 23 on side face 24 is positioned consistentwith the disposition of the louvers 5 a-5 d.

FIG. 6 shows the ventilation assembly 20 in a fully open configuration,in which the louvers 5 a to 5 d are all in fully open positions, i.e.horizontal as drawn. The set of control elements 23 on side face 24 arepositioned consistent with the disposition of the louvers 5 a-5 d.

FIG. 7 shows the ventilation assembly 20 in cross-section to better showthe full recirculation configuration of FIG. 5. Partitions 22 a and 22 chave a wedge shape profile such that when the louvers 5 a-5 d arepositioned at an appropriate angle as shown, a distal edge 30 (oppositeto the hinged edge 31) of each louver 5 abuts the wedge 33 at a wide endcorner 32 a or 32 b thereof. The louvers 5 a and 5 b each abut arespective corner of the wedge 33 a and the louvers 5 c and 5 d eachabut a respective corner of the wedge 33 b.

FIG. 8 shows the three positions of each louver 5 described so far.Position 80 a identifies the louver 5 in fully open (horizontal)position; position 80 b identifies the louver 5 in full recirculationposition; and position 80 c identifies the louver 5 in fully closedposition. The louvers 5 are each hinged at a hinged edge 31. FIG. 8 alsoshows one possible disposition of a fuel cell stack 82 adjacent thearray 6 of fans 7.

FIG. 9 shows a schematic view of a fuel cell stack assembly 40incorporating an alternative arrangement of ventilation assembly 41. Afuel cell stack 42 is disposed with in an air flow path 43 a, 43 bgenerated by fans 44 a, 44 b. The fuel cell stack comprises a stack offuel cells each of which has one or more cooling air conduits extendingtherethrough, and each cooling air conduit has input/output ventilationapertures disposed on a first ventilation face 49 a and on a secondventilation face 49 b of the stack. The ventilation apertures therebyform an array over each of the first and second ventilation faces 49 a,49 b of the fuel cell stack.

The fans 44 form an array similar to that described in connection withFIGS. 1 to 8 and each fan lies in an air guide chamber, a partition wallof which is seen at 45. It will be understood that the two fans 44 a, 44b visible may represent only a part of the array, which could extend inthe plane orthogonal to the drawing, and could have further fansabove/below those shown in the drawing. Each of the fans 44 a, 44 b isconfigured to direct air flow through a respective portion 50 a, 50 b ofthe ventilation face, each portion thereby corresponding to an area ofthe ventilation face 49 b covered by a respective fan. A louver panel 46includes a number of louvers 47 which can be varied in their angulardisposition or position so as to vary a flow of air through the louverpanel 46. In FIG. 9 a, the louvers 47 are shown in a fully open,non-recirculating position while in FIG. 9 b the louvers 47 are shown ina fully closed, full recirculation position. It will be understood thatthe louvers 47 could also be deployed in a partially open, partialrecirculation position where some air is allowed to pass through thelouvers. A filter box 48 is positioned in front of the fuel cell stack42.

FIG. 10 shows a schematic view of another alternative arrangement of afuel cell stack assembly 60 incorporating an alternative arrangement ofventilation assembly 61. A fuel cell stack 62 is disposed within airflow paths generated by forward fans 64 a, 64 b and reverse flow fans 82a, 82 b. The fuel cell stack 62 comprises a stack of fuel cells each ofwhich has one or more cooling air conduits extending therethrough, andeach cooling air conduit has input/output ventilation apertures disposedon a first ventilation face 69 a and a second ventilation face 69 b ofthe stack. The ventilation apertures thereby form an array over thefirst and second ventilation faces 69 a, 69 b of the fuel cell stack.The fans 64, 82 form two arrays similar to those described in connectionwith FIGS. 1 to 8. However, in this arrangement, the forward fans 64generally share an air guide chamber 65, without a partition wall 45 ofthe arrangement of FIG. 9. Similarly, the reverse fans 82 a, 82 bgenerally share a forward air guide or forward plenum 72. The twoforward fans 64 a, 64 b and the two reverse fans 80 a, 80 b that arevisible may represent only a part of the arrays, which could extend inthe plane orthogonal to the drawing, and could have further fansabove/below those shown in the drawing. Each of the forward fans 64 a,64 b is configured to generally direct air flow through a respectiveportion 70 a, 70 b of the ventilation face 69 b, although if the fansare operating at different rates, some air transfer via the air guidechamber 65 is possible. A louver panel 66 includes a number of louvers67 which can be varied in their angular disposition or position so as tovary a flow of air through the louver panel 66. In FIG. 10, the louvers67 are shown in a partially open, partial recirculation position wheresome air is allowed to pass through the louvers. The louvers 67 can alsobe deployed in a fully open, non-recirculating position or a fullyclosed, full recirculation position and all positions therebetween. Afilter box 68 is positioned in front of the fuel cell stack 62. Thereverse fans 82 a, 82 b may be lower power fans that the forward fans 64a, 64 b.

The operation of the various ventilation assemblies will now bedescribed.

First referring to FIG. 9 a, in first mode of operation, the ventilationassembly 41 ventilates the fuel cell stack 42 in a full,non-recirculating manner in which the fans 44 all draw air through arespective portion of the fuel cell stack 42 in the same direction asindicated by arrows 43 a and 43 b and the air is expelled through theopen louvers 47.

Maximal cooling is achieved (for a given fan speed). In a second mode ofoperation shown in FIG. 9 b, the louvers 47 are completely closed andthe upper fan 44 a is switched into reverse to direct air back throughthe stack 42 as depicted by reverse air flow path 43 c, while the lowerfan 44 b is maintained in a forward direction to draw air through thestack 42 in the forward direction depicted by air flow path 43 d. Theclosed louvers 47 ensure that the air flow 43 d that is directed throughthe portion 50 b of ventilation face 49 b by fan 44 b in a forwarddirection enters a plenum 51 and is then forced to return as air flow 43c in a reverse direction to be directed through the portion 50 a ofventilation face 49 b by the reverse operating fan 44 a. The partitionwall 45 ensures separation of forward and reverse air flows 43 d, 43 c.The reverse air flow 43 c might not require reverse operation of fan 44a. Some fan types do not run well or efficiently (or possibly do not runat all) in reverse direction and thus it is possible to rely solely onthe backpressure provided by the closed louvers 47 defining aclosed-ended plenum 51 to redirect the air flow, and switching off (orsubstantially reducing operation of) the fan 44 a. Minimal or reducedcooling is achieved in that a maximum or substantial proportion ofcooling air flow through the stack 42 has already passed through thestack once and is therefore somewhat preheated.

In a partial recirculation configuration, the louvers 47 are partiallyopen, e.g. disposed at an oblique angle (e.g. similar to the dispositionof louvers 67 shown in FIG. 10). By switching off (or substantiallyreducing operation of) one of the fans (e.g. 44 a), a proportion of theair flow 43 d from the portion 50 b of ventilation face 49 b can bedirected back as air flow 43 c, as the louvers 47 and cessation orslowing of fan 44 a create an increased back pressure. A portion of thestack 42 corresponding to the portion 50 a of ventilation face 49 b willreceive reduced cooling according to (a) the pre-heat of the reverse airflow, and (b) the reduction in total volume of air flow caused by thereduced back pressure of having the louvers 47 partially open. Thelouvers can adopt a plurality of possible intermediate configurationsbetween the open configuration and the closed configuration for forcingreturn of different proportions of air.

At the inlet end adjacent to filter 48, depending on the impedance toair flow provided by filter 48, the forward and return air flows 43 d,43 c may be mixed to a certain extent in a forward plenum 52 therebyresulting in repeat partial recirculation of air. This could be furthercontrolled by adjusting air flow impedance through the filter. If repeatrecirculation is not required, it can be avoided or substantiallyreduced by positioning the filter 48 immediately adjacent to theventilation face 49 a, i.e. eliminating forward plenum 52.

In a general aspect, it can be seen that the ventilation assembly 41provides, by way of the louver panel 46, a reconfigurable plenum 51 influid communication with a first fan 44 b and a second fan 44 a. Thereconfigurable plenum 51 has a first configuration (FIG. 9 a) in whichair is directed, by the first and second fans 44 b, 44 a, through firstand second portions 50 b, 50 a of the ventilation face 49 b in the samedirection. The reconfigurable plenum has a second configuration (FIG. 9b) in which air is directed, by at least one of the fans 44 b,respectively through the first and second portions 50 b, 50 a of theventilation face 49 b in opposing directions. This second configurationmay or may not be assisted by the reverse operation of fan 44 a. Theventilation assembly 41 also provides a plurality of intermediatepositions between these two extremes by control of the louver 47 angleand fan speed.

The second configuration can be reversed such that fan 44 a becomes theforward driving fan and fan 44 b is either switched off or driven inreverse. In this way, the portion of the stack that receives the primarycooling air flow and the portion which receives the subsequentpre-heated reverse air flow can be swapped, ensuring that temperaturecontrol can be controlled for all portions of the stack 42.

The ventilation assembly of FIG. 10 operates in a similar way but withsome differences. In first mode of operation, with the louvers 67 fullyopen, the ventilation assembly 61 ventilates the fuel cell stack 62 in afull, non-recirculating manner in which the fans 64 a, 64 b all draw airthrough respective portions 70 a, 70 b of the fuel cell stack 62 in thesame direction and the air is expelled through open louvers 67. Maximalcooling is achieved (for a given fan speed). In a second mode ofoperation, the louvers 67 are completely closed and the upper fan 64 amay be stopped, slowed or reversed. The closure of the louvers 67 causesbackpressure so that the pressure in air guide chamber 65 rises. Byactuation of reverse flow fan 82 a, at least a proportion of the airflow 63 b is directed back through the stack 62 as depicted by reverseair flow path 63 a. If the fan 64 a is stopped or slowed while fan 64runs at normal speed, then the fans 64 b, 64 a will also generate acontribution to backward flow 63 a, and the reverse fan 82 a may not berequired. Minimal or reduced cooling is achieved in that a maximum orsubstantial proportion of cooling air flow through the stack 62 hasalready passed through the stack once and is therefore somewhatpreheated.

In another mode of operation as shown in FIG. 10, the louvers 67 arepartially open and an air flow intermediate the two previously describedconfigurations will be effected. Increased back pressure from thelouvers 67 will slow the forward air flow through fans 64 a, 64 b.Operation of reverse fan 82 a will divert some of air flow 63 b acrossthe air guide chamber 65 so that a small reverse flow 63 a occursthrough the portion 70 a of ventilation face 69 b. Fans 64 a and 64 bcan be operated at differential speeds to encourage this reverse flow(i.e. fan 64 b operating faster than fan 64 a). Using separate reverseflow fans 82 a, 82 b allows use of fans that will not operate in reverseand also provides a greater degree of flexibility in varying air flowsin different parts of the fuel cell stack.

At the inlet end adjacent to filter 68, depending on the impedance toair flow provided by filter 68, the forward and return air flows 63 b,63 a may mix to a certain extent in a forward plenum 72 as indicated byair flow 73 thereby resulting in repeat partial recirculation of air.This could be further controlled by adjusting air flow impedance throughthe filter. If repeat recirculation is not required, it can be avoidedor substantially reduced by positioning the filter immediately adjacentto the fans 82 a, 82 b, i.e. eliminating forward plenum 72.

In a general aspect, it can be seen that the ventilation assembly 61provides, by way of a plenum 71 defined by the louver box 66, the airguide chamber 65 and the fans 82 a, 82 b, a reconfigurable plenum influid communication with a first fan 64 b and a second fan 64 a, inwhich the reconfigurable plenum has a first configuration in which airis directed, by the first and second fans 64 b, 64 a, through first andsecond portions 70 b, 70 a of the ventilation face 69 b in the samedirection, and a second configuration (louvers 67 at least partiallyclosed and reverse fan 80 a actuated) in which air is directed, by atleast one of the fans 64 b, respectively through the first and secondportions 70 b, 70 a of the ventilation face 69 b in opposing directions.The ventilation assembly 61 also provides a plurality of intermediatepositions between these two extremes by control of the louver 47 angleand by fan speed of at least the reverse fan 82 a.

The ventilation assembly 1 or 20 of FIGS. 1 to 8 can be assembledtogether with a fuel cell stack such as depicted in FIGS. 9 and 10, andoperates in a similar way but with some differences. In a first mode ofoperation depicted in FIG. 6, the ventilation assembly 1, 20 ventilatesa fuel cell stack in a full, non-recirculating manner in which the fans7 all draw air through a respective portion of the fuel cell stack inthe same direction and the air is expelled through the open louvers 5a-5 d (all of which are in position 80 a of FIG. 8). In a second mode ofoperation shown in FIG. 4, the louvers 5 are completely closed (in theposition 80 c shown in FIG. 8 and air flow through the stack can becompletely blocked. In a third mode of operation shown in FIGS. 5 and 7,the louvers 5 are in the recirculation position 80 b shown in FIG. 8.The fans 7 in one of the two banks 21 a, 21 b are switched off orswitched into reverse flow operation while the fans 7 in the other oneof the two banks 21 a, 21 b are maintained in a forward flow operation.The positioning of the louvers 5 in the recirculation position 80 b thenensures that forward air flow from bank 21 a is directed back into bank21 b and thereby directed back through the stack in similar manner tothat described in connection with FIGS. 8 to 10.

In a partial recirculation configuration, the louvers 5 are partiallyopen, i.e. allowing some air to pass through the louvers 5.

In a general aspect, it can be seen that the ventilation assembly 1, 20provides, by way of the louver panel 2, a reconfigurable plenum in fluidcommunication with a first fan 7 (bank 21 a) and a second fan 7 (bank 21b), in which the reconfigurable plenum has a first configuration(louvers in position 80 a) in which air is directed, by the first andsecond fans 7 in banks 21 a, 21 b, through first and second portions ofa ventilation face of a fuel cell stack in the same direction, and asecond configuration in which air is directed, by at least one of thefans 7 (e.g. one or more fans in bank 21 a), respectively through thefirst and second portions of the ventilation face in opposingdirections. The ventilation assembly 1, 20 also provides a plurality ofintermediate positions between these two extremes by control of thelouver angle and fan speed.

In all of the described embodiments, the parts of the fuel cell stackwhich are to be cooled with direct ambient air (from outside the system,e.g. as indicated by air flow 83 in FIG. 9), and those parts which areto be cooled with return air flow which is preheated, can be switched byappropriate selection of the fans 7, 44, 64 and louvers 5, 47, 67 to bedeployed and/or adjusted. The fans and louvers can be grouped in banks(e.g. banks 21) in which the fans in a bank or group all operate inconcert to direct air flows through selected portions of the stack inthe desired direction.

The louvers 5, 47, 67 could be replaced with any other form of closuredevice which can alter an air flow through a suitable housing defining aplenum, e.g. using shutters, iris diaphragms, etc. In a general sense,the reconfigurable plenum may be defined by any housing which includesany form of variable occluding member at an end of the plenum distalfrom the fans.

A suitable control mechanism may be used to automatically reconfigurethe system according to one or more operating parameters of the fuelcell stack. These parameters can include any one or more of thefollowing: operating temperature of the fuel cell stack or relevantportions of the fuel cell stack; ambient air temperature; temperature ofspecific air flows through the stack and/or through the ventilationassembly; individual cell temperatures; humidity levels in the stackand/or ambient air; cell or stack voltages; cell or stack currents; ortime of operation of the system. One or more physical environmentsensors may be disposed at strategic locations of the fuel cell assemblyand/or ventilation assembly to monitor any of these parameters.Preferably, reconfiguration of the system is controlled as a function ofoperating temperature or time or both. The reconfiguration of the systempreferably also sequences alternating sections of the stack toperiodically receive the direct cooler air and the returned warmer air.Thus, in a general sense, when operating in the second configuration,the system preferably periodically reverses the directions of air flow43 through the first and second portions 50 of the ventilation face 49.This periodic reversal may also be controlled as a function of operatingtemperature of at least a portion of the fuel cell stack and/or time ofoperation. The periodic reversal could be on a fixed or variablefrequency according to operating conditions of the fuel cell stack.

The array or arrays of fans may be individually sealed to the stack. Anarray may be dimensioned to comprise any appropriate number of fansdistributed along the planes of the cells in the stack and anyappropriate number of fans distributed across planes of the cells in thestack. A fan may straddle across any appropriate number of cells in thestack to define the portions 50 of a ventilation face 49. The fans maybe grouped in banks to cooperate with any particular style ofreconfigurable plenum.

It is preferable to configure the system such that alternating fans arearrayed along planes of the fuel cell stack such that individual fuelcells each have corresponding “warm” and “cold” sections rather thanhaving alternating fans arrayed across the planes of the fuel cell stacksuch that there are whole cells in stack which are hot and whole cellsin the stack which are cold, which may be more likely to cause failures.

Full closure of the cathode air flows can be effected by completeclosure of the louvers e.g. FIG. 4, which may alleviate ram air pressureconsequential on a moving vehicle in which the fuel cell is installed.Controlling air flows by full recirculation or closing louvers may alsobe used to assist in system shutdown.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. A fuel cell stack assembly comprising: a stack of fuel cells, eachfuel cell having a cooling air conduit with an input/output ventilationaperture disposed on a ventilation face of the stack, the ventilationapertures forming an array over said ventilation face of the stack; afirst fan configured to direct air flow through a first portion of theventilation face and a second fan configured to direct air flow througha second portion of the ventilation face; and, a reconfigurable plenumin fluid communication with said first fan and said second fan, theplenum having a first configuration in which air is directed, by thefirst and second fans, through the first and second portions of theventilation face in the same direction, and a second configuration inwhich air is directed, by at least one of the fans, respectively throughthe first and second portions of the ventilation face in opposingdirections.
 2. The fuel cell stack assembly of claim 1 in which theplenum is automatically reconfigurable as a function of operatingtemperature of at least a portion of the fuel cell stack and/or time ofoperation.
 3. The fuel cell stack assembly of claim 1 in which thereconfigurable plenum comprises: a first end proximal to said first andsecond fans; a second end distal to said first and second fansswitchable between an open configuration and a closed configuration;and, the open configuration facilitating exit of air flow from theplenum at the second end and the closed configuration forcing return ofat least some air from the first fan to the second fan.
 4. The fuel cellstack assembly of claim 3 in which the reconfigurable plenum isreconfigurable in a plurality of intermediate configurations between theopen configuration and the closed configuration each intermediateconfiguration forcing return of different proportions of air from thefirst fan to the second fan.
 5. The fuel cell stack assembly of claim 3in which the reconfigurable plenum includes a variable occluding memberat the second end of the plenum.
 6. The fuel cell stack assembly ofclaim 1 further including a fan controller configured to drive the firstand second fans in opposing directions when the reconfigurable plenum isin the second configuration.
 7. The fuel cell stack assembly of claim 1further including a fan controller configured to drive the first fan andshut down the second fan when the reconfigurable plenum is in the secondconfiguration.
 8. The fuel cell stack assembly of claim 1 furtherincluding a plurality of said first fans and a plurality of said secondfans, each of the first and second fans cooperating with a saidreconfigurable plenum.
 9. The fuel cell stack assembly of claim 8 inwhich the first fans and the second fans are arranged in groups, eachgroup cooperating with one said reconfigurable plenum.
 10. The fuel cellstack assembly of claim 1 in which the first portion of the ventilationface and the second portion of the ventilation face correspond todifferent parts of the same cells.
 11. The fuel cell stack assembly ofclaim 1 in which the first fan and the second fan are adjacent oneanother and adjacent the ventilation face.
 12. The fuel cell stackassembly of claim 1 further including a first reverse operating fanconfigured to direct air flow through the first portion of theventilation face in a direction opposite to that of the first fan and asecond reverse operating fan configured to direct air flow through thesecond portion of the ventilation face in a direction opposite to thatof the second fan.
 13. The fuel cell stack of claim 1 further includinga control system adapted to, when the system is operating in the secondconfiguration, periodically reverse the directions of air flow throughthe first and second portions of the ventilation face.
 14. A method ofoperating an air cooled fuel cell stack in which each fuel cell in thestack has a cooling air conduit with an input/output ventilationaperture disposed on a ventilation face of the stack, the ventilationapertures forming an array over said ventilation face of the stack, theventilation face having a first portion and a second portion, the methodcomprising: in a first mode of operation, ventilating the stack using afirst fan and a second fan, the first fan directing air through a firstportion of the ventilation face and the second fan directing air througha second portion of the ventilation face, the air flow through the firstand second portions being in the same direction; and, in a second modeof operation, ventilating the stack using at least the first fan todirect air through the first portion of the ventilation face in a firstdirection and through the second portion of the ventilation face in asecond direction opposite to the first direction.
 15. The method ofclaim 14 in which, in the second mode of operation, the stack isventilated using the first fan to direct air flow through the firstportion of the ventilation face in said first direction and using thesecond fan to direct the air through the second portion of theventilation face in said second direction opposite to the firstdirection.
 16. The method of claim 14 in which the operation of the fuelcell stack ventilation is switched between the first and second modes ofoperation by reconfiguring a reconfigurable plenum in fluidcommunication with said first fan and said second fan, thereconfigurable plenum having a first end proximal to said first andsecond fans and a second end distal to said first and second fans whichis switchable between an open configuration and a closed configuration,the open configuration facilitating exit of air flow from the plenum atthe second end and the closed configuration forcing return of at leastsome air from the first fan to the second portion of the ventilationface.
 17. The method of claim 14 further including the step ofautomatically switching between the first and second modes as a functionof operating temperature of the fuel cell and/or time of operation. 18.The method of claim 14 further including, when operating in the secondmode of operation, periodically reversing the directions of air flowthrough the first and second portions of the ventilation face.